1. Technical Field
The present invention relates to a laser beam source device, a lighting device, an image display apparatus, and a monitoring apparatus.
2. Related Art
Recently, a laser beam source device has been widely used in the field of light communication, light application measurement, light display, and other optoelectronics fields. The laser beam source device is divided into the type using wavelength of fundamental laser beam without change, and the type converting wavelength of fundamental laser beam before use. The latter type of the laser beam source device includes a wavelength conversion element for converting the wavelength of fundamental laser, for example. The wavelength conversion element is called second harmonic generation (SHG) element.
Generally, the conversion efficiency of the SHG is about 30 to 40%, and therefore the power of light converted by the SHG element is extremely smaller than the power of output light from the fundamental laser beam source. JP-A-59-128525 proposes a laser beam source device which has a structure for reducing power decrease of the output light. According to this laser beam source device, light emitted from an inside resonance type laser beam source passes through the SHG element. Then, the light is divided into first SHG light having converted wavelength and remaining fundamental light. Subsequently, the remaining fundamental light again passes through the SHG element to obtain second SHG light having converted wavelength. The second SHG light is combined with the first SHG light after converted into polarized light having polarization direction different from that of the first SHG light by 90 degrees. By this method, the laser beam source device according to JP-A-59-128525 reduces power decrease of the output light by using the combined light of the first SHG light and the second SHG light as the output light.
According to the laser beam source device described in JP-A-59-128525, the remaining fundamental light again passes through the SHG element to use the second SHG light after wavelength conversion. It is difficult, however, to use the remaining fundamental light which has wavelength not converted even after again passing through the SHG element. In this case, the use efficiency of light does not dramatically increase. Moreover, when this remaining fundamental light is returned to the fundamental laser beam source without change, there is a possibility that the power of the fundamental laser beam source decreases or becomes unstable. In this case, such a structure which prevents return of the remaining fundamental light to the light source is required, and thus the size of the optical system becomes larger. In addition, the length of the optical path increases or light needs to pass through the optical elements a larger number of times, which produces loss of light.
When the laser beam source device is combined with a liquid crystal device, a diffusion optical member for diffusing light from the laser beam source device is required. Light supplied to the liquid crystal device via a large-sized diffusion optical member has low image formability. Thus, compactness of the diffusion optical member is demanded.